1. Field of the Invention
The present invention relates generally to wireless frame transmissions.
2. Background Art
Wireless devices can transmit and receive data in the form of frames. A frame is a basic message unit for communication across a network, e.g., a wireless network. A frame can include routing information, data, and error detection information. Depending on the physical layer (PHY) protocol, a frame can be transmitted at various rates, e.g., the IEEE 802.11a PHY supports the rate set {6, 12, 18, 24, 36, 48, 54} Mbits/s. In general, the larger the distance between the transmitter and the target receiver, the lower the rate, since the received signal strength is lower. On the other hand, wireless communications are prone to interfering signals caused by, for example, microwave ovens, cordless telephones, Bluetooth devices, and other wireless devices, operating at overlapping or neighboring frequency ranges. In such noisy environments, the transmission rate of frames tends to be low in order to improve throughput and maintain a low packet error rate (PER). However, a low transmission rate does not necessarily provide higher throughput.
Conventional methods exist for choosing a wireless transmission rate in order to achieve a low PER. One such known method uses PER as the performance criteria. This method raises or lowers the transmission rate if the PER at the current rate meets a prespecified PER threshold. The thresholds can be specified per-rate, and they are set to maintain an overall low PER (e.g., around 5%). This method assesses PER at the current transmission rate and does not necessarily estimate PER at other rates. The PER-based method can suffer in performance in presence of interference for the simple reason that the interference can raise the PER to a level across all rates, and that level is not pre-known and can be higher than the preconfigured threshold.
Another known method uses the received signal strength indication (RSSI) to determine which transmission rate to use. RSSI tells the strength of an incoming (received) signal at a wireless device. The RSSI method assumes reciprocity in the medium, and both wireless devices at the two end points of a link use the same level of transmission power. Thus, no matter which device transmits, the other one observes the same strength of the received signal. However, the transmission power depends on many factors, including but not limited to device vendors, channel, and rate. Thus, the latter assumption is hard to guarantee. Decisions made solely based on RSSI with regard to the transmission rate may not be optimal.
As explained above, both PER-based and RSSI rate adaptation methods have their respective cons. Therefore, a system and method for adapting a wireless transmission rate to improve throughput in both clean and noisy environments are desired.